The invention relates to a device for measuring the mass of a flowing medium. There is already a known device (EP 0,547,595 A2) which possesses a tubular inner housing and a tubular outer housing and in which a temperature sensitive measuring element is accommodated in a central measuring duct of the inner housing, said measuring duct extending in the axial direction in the inner housing and being open on one side, in order to determine the mass of a flowing medium, in particular the intake air mass of an internal combustion engine. The device is provided as a mountable intermediate piece, for example of-an intake conduit through which the internal combustion engine can suck in air from the environment via an air filter. The tubular inner housing connected to the outer housing by means of a plurality of ribs possesses, furthermore, a bypass duct which is cut out from the inner housing and is arranged concentrically to the measuring duct and which, taking the form of an annular gap, surrounds the measuring duct with a smaller axial extension. A part stream of the medium flowing in the outer housing flows from an inlet mouth coaxial relative to the outer housing first into the measuring duct and flows round the temperature-sensitive measuring element arranged in the region of its downstream end, after which the flowing medium, reversing its direction of flow, flows from the measuring duct into the bypass duct. The medium flowing upstream in the bypass duct leaves the latter through a slit-shaped outlet orifice cut out on the circumference of the inner housing relatively far downstream of the inlet mouth and is mixed again with the medium flowing past between the inner housing and the outer housing. However, the design of the device as a mountable intermediate piece with an inner housing and with an outer housing necessitates a considerable overall size, so that the device is suitable to only a limited extent for confined conditions of installation, particularly in the engine region of a motor vehicle.
In the case of an internal combustion engine, as a result of the opening and closing of the inlet valves of the individual cylinders there occur considerable fluctuations or pulsations of the flow, the intensity of which depends on the intake frequency of individual pistons or on the rotational speed of the internal combustion engine. The pulsations of the flow are propagated from the inlet valves via the intake conduit as far as the measuring element in the inner housing and beyond this. The pulsations cause the measuring element, as a result of thermal inertia and directional insensitivity, depending on the intensity of the pulsations, to provide a measurement result which deviates considerably from the flow velocity prevailing on average in the measuring duct and from the intake air mass of the internal combustion engine which can be calculated from it.
In contrast to this, the advantage of the device according to the invention for measuring the mass of a flowing medium is that a uniformly accurate measurement result can be achieved virtually independently of a fluctuating or pulsating flow.
Advantageous developments and improvements of the device are specified herein after. It is Particularly advantageous that the device is distinguished by a compact design and small overall size and therefore requires only a small installation space. The device is therefore particularly suitable as a pluggable component especially for confined conditions of installation, for example in the engine region of a motor vehicle.